Afterburners (or secondary combustion chambers) are required for hazardous waste incineration units and kilns operating in this country to ensure the destruction of organic compounds which may be a result of incomplete combustion in the primary chamber of an incinerator. The afterburners are often as large or larger than the primary incineration chamber itself, and their operation can require a significant portion of the incinerator's fuel and operating costs. The batch or irregular charging or insertion of pollutants in wastes of varying types and varying amounts into the primary chamber of the incinerator can result in large transient "puffs" of incompletely combusted organic materials from the incinerator or kiln primary combustion chamber into the afterburner. The currently accepted practice requires an afterburner to provide an average residence time of 2 seconds at an average temperature of 1800.degree. F. Since proper mixing is a critical element of a well-operated incinerator, this approach does not guarantee the complete destruction of all products of incomplete combustion (PICs) during these transient excursions, and at the same time requires excess heating during periods when low levels of organic pollutants are present in the gases exiting the primary incinerator chamber.
The transient and unpredictable range of waste properties and pollutant levels leaving the kiln can lead to a lack of oxygen in the afterburner which will cause, among other things, fluctuations in the levels of carbon monoxide and hydrocarbon emissions from the incinerator. One method of overcoming these transient oxygen deficits is to inject oxygen into the afterburner. Oxygen is used rather than air to remove the necessity of heating the four parts of inert nitrogen introduced into the system for every part of oxygen when air is used. However, while the injection of oxygen into the afterburner can significantly reduce the previously described organic emissions, the additional cost of oxygen is too excessive to allow uncontrolled oxygen injection at high enough levels to handle all pollutant fluctuations.
Because it is impossible to accurately predict either the time of occurrence, or the magnitude, or the general shape (including rate of increase and fluctuations) of the transient increases in the emissions of PICs and thus the feed rate or amount of oxygen required, current control systems are forced to continually assume worst-case conditions as a control criteria to maintain acceptable stack emissions. System response to such variables are usually not known or predictable a priori. This leads to actual rates of oxygen injection (and therefore associated costs) which are much higher than the required rates. Traditional feedback controls have been based on the error or difference between a predetermined desired output and actual outputs. The traditional control system's response to such errors is then based upon some mathematical relationship between the measured state of the incinerator and the effects of changing one or more operating parameters. However, this relationship must incorporate detailed knowledge of the physical and chemical mechanisms which govern the behavior of the system. If such mechanisms are not known, the relationship must be significantly simplified, thereby overlooking potentially important effects. Since the relationships between changes in operating parameters and the state of the incineration system are usually not known, traditional control systems must rely on constant operator inputs to maintain the desired conditions within the incinerator. These constant adjustments can result in considerable differences in operation as different operators react to changes within the incinerator, potentially leading to increases in both the emissions of pollutants and the amount of fuel consumed.
Fuzzy logic-based control does not require a priori determination of relationships needed for traditional feedback control systems. Fuzzy logic control rules are based on heuristic operating relationships which are analogous to, and which approach, the intuitive response of a human operator thus providing a more flexible and responsive intelligent feedback control system than is possible with conventional prior art afterburner pollutant control systems. The use of fuzzy logic enables the use of "if-then" rules applied to varying degrees, similar to the way system operators control a system. An operator, rather than using an on-off control approach, may note a low oxygen level and increase oxygen flow "a little bit". Such behavior is simulated by fuzzy logic.
Fuzzy logic provides a less rigid control scheme for control of the incinerator processes that are not easily defined, and provides good control response for unforeseen circumstances that cannot be programmed into a rigid control system design.
The theory of fuzzy logic and an explanation of its ability to be utilized in complex, ill-defined systems, is set forth, for example, in the article "Designing with Fuzzy Logic" by Kevin Self, p. 42, IEEE Spectrum, November 1990, which is hereby incorporated by reference. Fuzzy logic has been applied to various complex control systems. Examples include the use of fuzzy logic for optimization of the efficiency of electric motors to reduce electric power consumption by varying the motor speed to match varying load requirements while at the same time minimizing power input. See U.S. Pat. No. 5,272,428 entitled "Fuzzy Logic Integrated Control Method and Apparatus to Improve Motor Efficiency, of Paul J. Chappell and Ronald J. Spiegel, assigned to the same assignee as the present patent application, and hereby incorporated by reference.